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Documentation Index

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This guide covers the fundamentals of lithium-ion batteries. Whether you’re new to battery science or just looking to brush up on the basics, this section will guide you through the inner workings of these devices that power our everyday lives—from phones and laptops to electric vehicles.

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What’s Inside a Lithium-Ion Battery?

If you were to open a lithium-ion battery (please don’t try this at home under any circumstances), you’d find that it contains either a very long and thin sheet rolled up (in cylindrical or prismatic batteries) or several thin sheets stacked on top of one another (in pouch batteries). Looking closer, we’d notice that these sheets consist of several layers stacked together:

Positive Electrode

Often referred to as the cathode. This is a porous layer typically made of lithium-metal oxide particles (e.g., lithium cobalt oxide or lithium iron phosphate) “glued” together by some additives.

Negative Electrode

Often called the anode, it is similar to the positive electrode but typically made of graphite.

Separator

A thin, porous membrane that keeps the positive and negative electrodes apart to prevent short circuits, while allowing ions to pass through.

Electrolyte

A liquid or gel that fills the pores within the electrodes and separator, facilitating the movement of lithium ions. It usually consists of a lithium salt dissolved in an organic solvent.
Additionally, current collectors (thin metal foils—typically aluminum for the positive electrode and copper for the negative one) connect the electrodes to the external circuit.
This combination of components, often referred to as a cell, forms the basic electrochemical unit capable of storing and delivering energy.

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How Does a Battery Work?

At the heart of a battery’s operation lies the movement of lithium ions and electrons between the electrodes. Here’s a simplified explanation of the process:
1

When Fully Charged

Lithium ions are stored (or “intercalated”) within the layers of the negative electrode. This is the battery’s high-energy state, ready to deliver power.
2

Discharge

When the battery is connected to a device, the external circuit is closed and electrochemical reactions occur at both electrodes. At the negative electrode, lithium deintercalates and releases electrons. These electrons flow through the external circuit, powering the device. Simultaneously, lithium ions travel through the electrolyte to the positive electrode, where they intercalate along with electrons arriving from the external circuit.
3

Charge

By applying an external voltage to the battery, the process is reversed. Lithium ions move back to the negative electrode, restoring the battery to its high-energy state.
This reversible movement of lithium ions and electrons is what makes lithium-ion batteries rechargeable.
Of course, the process also involves additional phenomena such as gas formation, electrode swelling, and solid-electrolyte interphase (SEI) growth, which are covered in the degradation section.